Finding Speaker Dead Spots with Lasers

In a curious combination of technology, researchers at the National Physical Laboratory have devised a way to make pictures and even video of sound waves as they are generated by a speaker. Many modern speakers suffer from dead spots, where the sound waves it emits will cancel each other out. Manufacturers look for these spots by moving a microphone around a speaker during an experiment, or by using computer simulations, which are not always correct. What NPL has done is aimed a laser past the front of a speaker. Light is affected by the density of the air it passes though, and sound waves change the density of air. This acousto-optic effect produces a phase change in the light which can be measured to produce a map of the sound waves. This technique can have an impact on the speakers used for outdoor events as well as inside our homes.